Why? To donate the shorn hair to a charity that gives wigs to women battling cancer.

“On a personal level, it was kind of hard to top as an adrenaline rush,” Lane, sporting a sleek, brand-new bob, told The Associated Press. “But, really, it’s a great relief to know it’s not really about the hairdo I’m left with. It’s more about where the hair that’s gone went.”

More celebrities are giving millions, even billions, to charity and taking up humanitarian causes. They’re also drumming up media attention to get the message out — and that’s not to mention their work for political candidates and their campaigns.

What’s the motive for this high-profile munificence? It’s a mix of factors, underpinned by the viewpoint that philanthropy is a must on a celebrity’s to-do list.

“I think we’re finding a new era in which as more do it, more will put it on their agenda,” said Paul Schervish, director of the Boston College Center on Wealth and Philanthropy. “That is, they will be responding to social expectations.”

Those expectations are being set by A-listers such as Angelina Jolie, a goodwill ambassador for the U.N. High Commissioner for Refugees, whose passion for the plight of refugees and other issues has helped encourage celebrity activism, said Marc Pollick, the director of The Giving Back Fund, a nonprofit organization that mentors celebrities about philanthropy.

Jolie and beau Brad Pitt, perhaps the world’s foremost activist couple, sold the first picture of daughter Shiloh to People magazine, saying all proceeds would be donated to charity. They recently gave $300,000 to help government-run hospitals in Namibia, where Shiloh was born.

When Jolie, who uses her star wattage to lure the media’s spotlight to impoverished corners of the globe, gets praised for her efforts, her peers feel compelled — even pressured — to do the same.

“Others are saying, ‘Gee, this is a great thing to do and look at all the good it produces,'” Pollick said. “And it’s good on a number of levels. And so I think the copycat behavior is fabulous. It’s sort of a tide rising all the boats.”

Nicolas Cage recently pledged $2 million to help former child soldiers around the world. Action star Jackie Chan intends to dispense half his wealth — that’s $128 million, he estimates — to charity.

Oprah Winfrey is the only show-biz celebrity to rank, at No.22, on Slate.com’s 2005 list of the top 60 donors to charity.

Celebrity activism, however, is nothing new.

Jerry Lewis has hosted his Muscular Dystrophy Association telethon for 40 years. Paul Newman, through his Newman’s Own line of food products, has raised more than $200 million for charity since 1982. (And for years, good deeds have been done in stealth.)

“This is a kind of philanthropy that existed a long time, but as all forms of philanthropy now are becoming more prominent — donations and the people involved are becoming more substantial — this has taken on a life of its own and has become something that celebrities are compared with each other about how much they’re doing,” Shervish said.